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to the Kalam Cosmological Argument is that it requires an A Theory of time, but most physicists reject the A Theory. To explain why, I explained and then Einstein’s theory of .

Hopefully you remember that special relativity merely asserts that the laws of physics work the same for all reference frames in uniform motion; that is, a microwave oven on a distant planet moving very fast relative to Earth.

That seems obvious. So why did it take a genius like Einstein to come up with special relativity? On reason is that the implications of this simple hypothesis are very strange, and Einstein’s stroke of insight occurred when he was able to throw out his intuitions about time and follow the datainstead.

The speed of light

According to Einstein, the laws of physics, and therefore the speed of light, must be the same for all reference frames; for all observers (from here on out, I’ll drop the “in uniform motion” bit and take it to be assumed).

But if all observers will measure the same speed for light (speed c), then even two observers that are moving relative to each other will observe the same speed for light. And that’s where it gets weird.

Remember my story about the . If you’re standing still and I drive toward you in a car and shoot you with a Nerf dart gun, the dart will hit you with greater speed than if I had shot you while standing next to you. And if I pass you and look back and shoot you with the Nerf dart while driving away from you, then (assuming no air resistance and that my gun is powerful enough) the dart will hit you with less speed than if I had shot you while standing next to you.

But that’s not true for light. Let’s say you’re tethered to a space station in deep space, and I fly my spaceship toward you at 100 million miles per hour with my headlights on. The light from my headlights will go past you at 671 million miles per hour, or “speed c.” But if I park my ship next to you and turn on the headlights, that light will still go by you at speed c. And if I zoom past you at 100 million miles per hour and shine a light back at you, that light will go by you at speed c, too.

That seems really weird. How can that be?

It’s a simple consequence of the fact that the laws of physics are the same for all observers, and the speed of light is one of the laws of physics. (The speed of a Nerf dart is not one of the laws of physics.)